1. Field of the Invention
The present invention relates to a valve operating apparatus for operating intake (or inlet) valves and/or exhaust valves of an internal combustion engine, the valve operating apparatus comprising valve-operating cams and rocker arm units. The present invention relates, in particular, to a valve operating apparatus in which each of the rocker arm units is made up of driving rocker arms which operatively contact valves, a free rocker arm which is free from operating contact with, or does not operatively contact, the valves, and a changeover mechanism which varies the valve lift amount and the timing of opening and closing the valves by connecting the rocker arms and releasing their connection.
2. Description of Related Art
In this kind of valve operating apparatus, when the connection between the free rocker arm and the driving rocker arms is released, an urging force of a valve spring will no longer function or operate on the free rocker arm. Therefore, it is necessary to provide an urging means to urge the free rocker arm into operating contact with a valve driving cam.
Conventionally, as shown in FIG. 16, a bridge member c is provided between cam holders b which support rocker arm shafts "a" of rocker arm units, and an urging means d which comprises a piston d.sub.2 to be urged by a spring d.sub.1 is mounted on the bridge member c such that the piston d.sub.2 operatively contacts a free rocker arm e, wherein the free rocker arm e is urged towards the valve driving cam f (see Japanese Published Unexamined Patent Application No. 1405/1992). Alternatively, as shown in FIG. 17, the urging means d having a similar construction as described above is mounted on a boss portion g which is provided in a projecting manner on a cylinder head, such that the piston d.sub.2 operatively contacts the free rocker arm e, whereby the free rocker arm e is urged towards the valve driving cam f.
The conventional example shown in FIG. 16 has the following disadvantages. Namely, the bridge member c for mounting thereon the urging means d becomes necessary, resulting in an increase in weight and cost. Further, a space for disposing the urging means d must be provided above the rocker arm unit, resulting in an increase in height of the engine.
The conventional example shown in FIG. 17 also has the following disadvantages. Namely, the boss portion g for mounting thereon the urging means d must be provided in a projecting manner on the cylinder head, resulting in an increase in the weight. Further, a space for disposing the urging means d must be provided between the adjoining valves, resulting in a limitation, or a restriction, in the freedom in the design of the valve layout.
In any of the above-described conventional examples, there is a further disadvantage in that the construction of the urging means d becomes complicated, resulting in a higher cost.